1. Field of the Invention
The present invention relates to a method of heating a thermoplastic resin sheet or film. More particularly, it relates to a method of preheating, heating or heat-treating a thermoplastic resin sheet or film for obtaining a desired product by monoaxial drawing, biaxial drawing, heat setting, coating or laminating operations, in which the sheet or film is uniformly heated in a highly precise manner and with a specific temperature profile in the lateral direction of the sheet or film.
2. Description of the Prior Art
When a desired product is prepared from a thermoplastic resin sheet or film by the above-mentioned operation while preheating, heating or heat-treating the sheet or film at a predetermined temperature, an indirect heating method is adopted. A hot air heating method can be mentioned as an example of this indirect heating method. According to this hot air heating method, heated air is blown against a sheet or film to be heated. When this heating method is adopted, the temperature of the sheet or film readily becomes very close to the temperature of heated air, and therefore, the temperature can be adjusted relatively easily, the influence of the ambient temperature outside a heating furnace is small and the temperature precision is high. However, in this method, the sheet or film is likely to wave become wavy, and a partial distortion is caused in a sheet or film heated at a temperature close to the softening temperature of the resin, especially in the heated film. Namely, so-called flaring is caused, and when the product is unwound, waves are formed on the film surface and the quality of the product is degraded. Moreover, in the case where the film is very thin, breaking is caused because of this waving.
As another indirect heating method, there can be mentioned a radiation heating method in which a sheet or film is heated by irradiating the sheet or film with electromagnetic waves of infrared or far infrared ray regions. This heating method is advantageous in that (a) the heating furnace can be constructed at a low cost, (b) the maintenance of the heating furnace is easy and (c) the heating efficiency is high. However, the method is defective in that (d) the heating furnace is readily influenced by a power source voltage applied to heaters arranged in the furnace and (e) the heating temperature is readily influenced by the outer atmosphere surrounding the heating furnace, and therefore, the heating precision in the sheet or film is low.
The principle of the heat conduction in heating a sheet or film according to the radiation heating method is illustrated in the conceptual diagram of FIG. 1. Referring to FIG. 1, even if the temperature TH of a heater 12 is maintained at a constant level, when the ambient air temperature TA is changed, the temperature TF of a film 11 is influenced and changed. Accordingly, in order to maintain the film temperature TF at a constant level, it is necessary to occasionally correct the set value of the heater temperature TH.
In the case where a sheet or film is heated according to the radiation heating method, a plurality of heaters arranged in the radiation heating furnace interfere with one another. This interference state is shown in the conceptual diagram of FIG. 2, in which reference numerals 22, 23, 24 and 25 represent a heater, a heat reflecting plate, a clip and a clip rail, respectively.
Referring to FIG. 2, at a position i on a sheet or film 21 (positions located on the right side of the position i in the lateral direction on the sheet or film 21 are designated as i+1, i+2, . . . , and positions located on the left side of the position i in the lateral direction on the sheet or film 21 are designated as i-1, i-2, . . . ), the radiation heat from a heater located at a position j just above the position i (the positions i, i+1, i+2, i-1 and i-2 on the sheet or film correspond to the heater positions j, j+1, j+2, j-1 and j-2, respectively) is largest at the position i on the sheet or film 21, but radiations from the positions j-2, j-1, j+1 and j+2 cannot be neglected. Accordingly, even if only the set value of the temperature of the heater at the position j just above the position i is changed so as to adjust the temperature at the position i on the sheet or film 21, the temperature precision throughout the lateral direction of the sheet or film is not improved.
In the case where a biaxially drawn film of a thermoplastic resin is prepared while adopting the radiation heating method, the film temperature in the central portion of the heating furnace becomes higher than the film temperature in the vicinity of the heating furnace wall. As means for adjusting the temperature in a highly precise manner as in this case, there are known (a) a method in which a plate-like shielding member having many through holes with different diameters is arranged between a heater and a film at a position where the film is readily over-heated, and (b) a method in which a plurality of nets are arranged instead of the plate-like member having holes with different diameters, used in method (a). Even if these methods (a) and (b) are adopted, delicate temperature adjustment is difficult, and it is not easy to appropriately change the position, hole diameter and number of the interposed shielding member according to the conditions. Moveover, since the interposed shielding member cuts heat waves, the heating efficiency is reduced.